The present invention relates to an optical information recording medium for recording information at high density by using a high-energy beam such as a laser beam, etc.
A technology is known in which a photosensitive recording layer is formed on a flat surface of a transparent substrate and a laser beam is irradiated onto the recording layer through the substrate so as to produce a change at an irradiated portion of the recording layer such that information is recorded at the irradiated portion. Such a known recording medium is described with reference to FIG. 1. In FIG. 1, the known recording medium includes a pair of substrates 2. A plurality of guide grooves 6 for sequentially guiding a laser beam for recording and reproducing signals to locations for recording are formed on one face 1 of each of the substrates 2. A recording thin-film layer 3 is formed on the face 1 and the substrates 2 are bonded to each other through the recording layers 3 by an adhesive layer 4.
The substrate 2 is formed by a plate made of glass, resin, etc. The recording thin-film layer 3 is formed by a film made of metal having a low melting point, organic coloring matter, amorphous alloy, etc. A technique has been proposed in which rewritable recording is performed by employing as the recording thin-film layer 3 of amorphous alloy. For recording, a method is used in which phase change between amorphous phase and crystalline phase is caused such that change in reflectance therebetween is produced, or a method is used in which the direction of magnetic spin of the magnetic film directed perpendicularly to the substrate can be changed such that reproduction is performed by a magneto-optical effect. In such medium, a protective layer made of dielectric material is provided on the surface of the recording layer or a metallic reflection layer is provided for the purpose of improving absorptance of the recording layer. The known recording medium shown in FIG. 1 includes the two substrates 2. However, a single substrate 2 may be employed in the recording medium.
The substrate 2 usually has a thickness of 1.2 mm. This is because an objective lens for converging a laser beam upon the recording film usually has a numerical aperture of about 0.5 or less in practical use. In FIG. 2, assuming that an objective lens 7 has a numerical aperture NA, an effective diameter D, a focal length f and a working distance WD, the following relation (1) is established. EQU f=D/2NA&gt;WD (1)
The above equation indicates that a predetermined limit exists between the focal length f of the objective lens 7 and the thickness t of the substrate 2. Namely, if the thickness t of the substrate 2 is increased while keeping the effective diameter D and the numerical aperture NA of the objective lens 7 to the laser beam 8 which enters into the medium from the substrate side, the working distance WD becomes insufficient and thus, the objective lens 7 may come into contact with the substrate 2. On the contrary, if the thickness t of the substrate 2 is reduced while keeping the effective diameter D and the numerical aperture NA of the objective lens 7, the possibility of the above mentioned inconvenience of contact of the objective lens 7 with the substrate 2 is lessened but the difference between the spot diameter d1 of the laser beam 8 on the surface of the substrate 2 and the spot diameter d2 of the laser beam 8 on the recording film 3 is decreased. As a result, if foreign matter such as dust is present on the substrate 2, quantity of light on the recording film 3 is reduced greatly, thereby resulting in a drop of recording sensitivity or reproducing signal quality. In prior art recording apparatuses having a numerical aperture of about 0.5, a substrate having a thickness of 1.2 mm is employed for the foregoing reasons.
In order to raise recording density higher than that of the prior art recording apparatuses, a procedure may be employed in which convergence of the laser beam is improved by increasing the numerical aperture of the objective lens. In this procedure, it is not practical that the size of an optical recording head be made larger than that of the prior art recording apparatuses. Thus, if an objective lens having a numerical aperture approximately equal to that of the known recording apparatuses is employed, the focal length of the objective lens becomes smaller than that of the prior art recording apparatuses as will be seen from the equation (1). In the case of the hitherto employed substrate 2, the focal surface will be inside the substrate, so that the objective lens having a large numerical aperture cannot be employed. Accordingly, if a objective lens having a large numerical aperture is employed, it becomes necessary to employ a substrate having a thickness smaller than that of the prior art recording apparatuses. For example, if the numerical aperture is set at 0.65 and 0.75, the substrate is required to have thicknesses of about 0.6 mm and 0.3 mm, respectively. However, when thickness of the substrate is merely reduced, the following problems arise. For example, in the case where the substrate is made of resin, the mechanical strength of the substrate drops, so that warpage, deflection, etc. of the substrate occur at the time of formation of the recording film on the substrate and thus, run-out of the surface of the recording medium occurs when the recording medium is rotated for recording or reproduction. If the substrate is made of glass, warpage etc. of the substrate are lessened slightly but a drawback occurs that the substrate is readily cracked when an impact occurs on the substrate.